1. Field of the Invention
The present invention relates to a development device used in image formation adopting an electro-photographic process or an electrostatic recording process.
2. Description of the Related Art
As the development device used to develop an electrostatic latent image formed on an image bearing member by the electro-photographic process or the electrostatic recording process, by a one-component developer or a two-component developer, a large number of development devices including a development sleeve have been conventionally discussed and adopted.
In general, the development sleeve is supported in an opening part via bearings at both end portions to be freely rotatable. The development sleeve has a surface subjected to surface roughing processing by using a blast or the like. Alternatively, the development sleeve has a surface on which a groove formed in a lengthwise direction is arranged regularly in a circumference direction. The surface of the development sleeve bears and conveys the developer. The latent image on the image bearing member is visualized by the borne developer.
In a case where the quantity of the developer on the surface of the development sleeve is non-uniform, the visualized image on a photosensitive drum also becomes non-uniform in concentration, resulting in a problem on the image. Therefore, it is desirable to make the quantity of the developer on the surface of the development sleeve, uniform. It is generally conducted to uniformly regular the quantity of the developer on the surface of the development sleeve by using a regulation member called regulation blade.
Furthermore, the development device includes a development container to accommodate the developer. In general, conveyance members, such as a screw, are disposed in the development container. The developer is circulated and conveyed in the development container by these conveyance members.
In recent years, speed increase has been promoted in such image forming devices using the electro-photographic process. In a case where the rotary moving speed of the photosensitive drum is comparatively low, a sufficiently favorable developed image is obtained even if the development time is short. Even if one development sleeve is used, therefore, a favorable image is obtained.
In a case where the rotary moving speed of the photosensitive drum is increased, however, a suitable image forming cannot be always conducted with one development sleeve. As a countermeasure, there is a method for improving the development efficiency by increasing the peripheral speed of the development sleeve. If the peripheral speed of the development sleeve is increased, however, centrifugal force acting upon the developer that forms a magnetic brush increases, and scattering of the developer increases. As a result, contamination within a copying machine is caused, and the function of the device might be lowered.
As a different countermeasure, therefore, a so-called multi-stage magnetic brush development method using two or more development sleeves is discussed (Japanese Patent Application Laid-Open No. 2007-72221). In the multi-stage magnetic brush development method, a plurality of development sleeves is disposed to be adjacent to each other with peripheral surfaces in close proximity to each other. The developer is conveyed continuously on peripheral surfaces to prolong the development time and enhance the development capability.
In such magnetic brush development having, for example, two development sleeves, however, there is a problem that the developer leaks out from between an upstream development sleeve and a downstream development sleeve at a development sleeve end portion into the rotation direction of the downstream development sleeve. Japanese Patent Application Laid-Open No. 2007-72222 takes a countermeasure against this problem. In each of end portions of the development sleeve in an axial direction, magnet members are disposed in close proximity in a non-contact state without being facing a delivery magnetic pole. Among the magnet members, at least a surface facing on the downstream side in the rotation direction of a downstream development sleeve is made the same polarity as a delivery magnetic pole of the downstream development sleeve. It is said that owing to such a configuration, leak of the developer from between the upstream development sleeve and the downstream development sleeve can be suppressed.
If the peripheral speed of the development sleeve is increased in accordance with the request for further increase of the speed, however, leak of the developer, which cannot be neglected, occurs from between two sleeves depending upon a position relation between a repulsion pole of the downstream development sleeve and the magnet member.